The gas leak from a hole formed by corrosion, etc. in a gas pipe not exposed outside like a buried pipe in the piping system of a city gas supply network is conventionally inspected by using various methods such as the sense of smell or a gas detector, or boring work in combination with the foregoing means, etc. If gas leak is detected in reference to the smell or in periodical inspection, the ground is dug or the wall is removed, to exchange or repair the gas pipe.
However, in the conventional methods, since any component of the gas itself or an odorous component added beforehand to the gas which leaks and diffuses from a damaged portion of the gas pipe is detected by using a gas detector or the human sense of smell, to identify the fact of leak and the location of leak, the occurrence of gas leak and its location cannot always accurately detected.
For example, if a cavity is formed in parallel to a gas pipe installed underground or inside a wall, etc., it can happen that the gas leaking from a damaged portion of the gas pipe migrates through the cavity and diffuses into air or indoor, etc. at a position apart from the damaged portion, and in this configuration, it can happen that the position apart from the actually damaged portion is detected as the damaged portion by error. If excavation or wall removal, etc. is effected as mentioned above based on the wrong information, the work is wasteful, and second and even third work may be required disadvantageously. Furthermore, if the gas leaking from a damaged portion of a gas pipe is retained near the damaged portion without diffusing in air or indoor, etc., the leak itself may not be able to detected.
As described above, the method of inspecting the occurrence of leak and the damaged portion by detecting any component of the gas itself or the odorous component added beforehand to the gas leaking and diffusing from the damaged portion cannot always accurately detect the occurrence of leak and its location.
On the other hand, one of conventional general inspection methods for detecting the damage of metallic structures is ultrasonic flaw detection, but it is difficult to apply this method for inspection of piping systems not exposed, for such reasons that the probe must be kept in tough with the object to be inspected, and that flaw detection for a long distance cannot be made.
An object of the present invention is to overcome the above mentioned disadvantages of the conventional inspection methods, and to provide an inspection method which can relatively accurately detect damaged portions of pipes such as holes formed by corrosion, when applied to the inspection of elements of piping systems, particularly metallic piping systems not exposed.
Another object of the present invention is to provide an inspection which can measure the distances of piping systems to damaged portions and can detect the locations and states of some joints as other elements of piping systems, in addition to the detection of damaged portions of pipes.